Lithium alloy/iron sulfide batteries, currently under development, have positive and negative electrode materials confined by or relative to structural positive and negative current collectors, which are electrically insulated from one another by separators. Typically, the negative electrode material is a lithium alloy (generally LiAl), the positive electrode material is an iron sulfide (FeS or FeS.sub.2), and the separators are formed of a fibrous boron nitride (BN) or a pressed powder magnesium oxide (MgO). An electrolyte such as a lithium chloride and potassium chloride mixture (LiCl-KCl), is normally infiltrated into the electrode materials and into the separators. The positive and negative current collectors are commonly formed of a conductive open mesh-like sheet or plate construction so as to confine the electrode materials while also allowing the migration of the electrolyte as required relative to the confined electrode materials. Existing commercially fabricated full size batteries of this type are comprised of many cells, each having the construction noted above, that are housed together in a common battery housing and that are electrically connected in series to produce higher effective voltage output.
This type of battery or cell is designed to operate at temperatures in the range of 425.degree.-500.degree. C. The electrode materials and electrolyte are most corrosive at these temperatures so that the current collectors must be of corrosive resistant yet electrically conductive material. Some success has been obtained by using stainless steel clad over copper. Moreover, the battery is designed to have an operating life in excess of 1000 "deep discharge" cycles, where each "deep discharge" cycle means discharging the fully charged battery down to approximately only a 20% charge level before recharging it again. During this deep discharge cycling, the positive and negative electrode materials undergo volumetric changes at different rates. This can shift the physically confining respective current collectors relative to one another within the battery cell or can even deform the collectors and/or cell housing. Also, nonuse of this type battery allows the operating temperatures of the electrolyte and electrode materials (each a paste-like liquid at the operating temperatures) to drop to temperatures whereat they can freeze solid. These freeze-thaw cycles can also cause movement between the current collectors, electrode materials and cell housing.
Although the shifting movement or the deformation may only be minor, it can be sufficient to cause a short in the battery, particularly over an extended number of cycles. This results in a decline in the coulombic efficiency, and a battery might be considered marginal when its coulombic efficiency is reduced to 95%.
The most common source of a cell short is the direct contact of the current collectors with one another. Another common cause of a cell short is where one electrode material oozes from its constraints and bridges to the opposite electrode material. Another form of cell short occurs where the positive electrode material "swells" out more than the negative electrode material to short against the cell housing, which most commonly is at the negative potential. Efforts to reduce these problems by fixturing, etc. the current collectors relative to one another or by reinforcing the cell housing by external constraints have to date only been marginally successful.
Another major problem in existing battery designs has been the number of separate structural plates that must be used in the cell to form the current collectors, and the number of separators that must likewise be used. These structural components generally must be sequentially fixtured in place relative to one another to define the sandwiched assembly. This piece-work fabrication requires extreme accuracy and care. Moreover, pretesting of these stacked plates and separators for possible shorts prior to positioning them within the cell housing is difficult, or meaningless, inasmuch as they yet could be shifted before ending up in the cell housing. The active electrode materials frequently are assembled simultaneously with the structural components being assembled, even though the presence of these chemical components made the fabrication more difficult, time consuming, costly and unreliable.